In wellbores, annular barriers are used for different purposes, such as for providing a barrier for flow between an inner and an outer tubular structure or between an inner tubular structure and the inner wall of the borehole. The annular barriers are mounted as part of the well tubular structure. An annular barrier has an inner wall surrounded by an annular expandable sleeve. The expandable sleeve is typically made of an elastomeric material, but may also be made of metal. The sleeve is fastened at its ends to the inner wall of the annular barrier.
Multiple annular barriers may be used to seal off a zone between an inner and an outer tubular structure or a well tubular structure and the borehole. A first annular barrier is expanded on one side of the zone to be sealed off, and a second annular barrier is expanded on the other side of that zone, whereby the zone is sealed off.
An annular barrier may be set using a pressurised fluid which is injected into the well or into a limited part of the well. Hereby, the expandable sleeve of the annular barrier is expanded to engage with an outer tubular structure or the inner wall of the borehole. The pressure envelope of a well is governed by the burst rating of the tubular and the well hardware etc. used within the well construction. When the expandable sleeve is expanded by increasing the pressure within the well, the burst rating of a well defines the maximum pressure that can be applied. It is desirable to minimise the expansion pressure required for expanding the sleeve to minimise the exposure of the well to the expansion pressure.
To reduce the expansion pressure of the annular barrier, the thickness of the expandable sleeve may be decreased. However, this impairs the strength of the expandable sleeve and the maximum expanded size of the sleeve. Further, the sleeve may collapse or rupture before the desired expanded size of the sleeve is reached. A frequently occurring reason for ruptures of expandable sleeves is inexpedient thinning of the sleeve material during expansion. Thinning of the sleeve material is an important property of the expandable sleeve, but too much thinning, e.g. in a local region of the sleeve, will cause the annular barrier to malfunction.